Gluten Free Chocolate Chip Cookies

24 ounces (4 1/2 cups plus 1/3 cup) white rice flour
7 1/2 ounces (1 2/3 cups) brown rice flour
7 ounces (1 1/3 cups) potato starch
3 ounces (3/4 cup) tapioca starch
3/4 ounce (1/4 cup) nonfat dry milk powder

Whisk flour (or flour blend), baking soda, xanthan gum, and salt together in medium bowl; set aside. Whisk melted butter, brown sugar, and granulated sugar together in large bowl until well combined and smooth. Whisk in egg, milk, and vanilla and continue to whisk until smooth. Stir in flour mixture with rubber spatula and mix until soft, homogeneous dough forms. Fold in chocolate chips. Cover bowl with plastic wrap and let dough rest for 30 minutes. (Dough will be sticky and soft.)
Adjust oven rack to middle position and heat oven to 350 degrees. Line 2 baking sheets with parchment paper. Using 2 soup spoons or a #50 cookie scoop, use 1 1/2 tablespoons dough at a time, portion dough and space 2 inches apart on prepared sheets. Bake cookies, 1 sheet at a time, until golden brown and edges have begun to set but centers are still soft, 11 to 13 minutes, rotating sheet halfway through baking.
Let cookies cool on sheet for 5 minutes, then transfer to wire rack. Serve warm or at room temperature. (Cookies are best eaten on the day they are baked, but they can be cooled and placed immediately in an airtight container and stored at room temperature for up to 1 day.)